Exchange Reactions between Alkanethiolates and Alkaneselenols on Au{111}

When alkanethiolate self-assembled monolayers on Au{111} are exchanged with alkaneselenols from solution, replacement of thiolates by selenols is rapid and complete, and is well described by perimeter-dependent island growth kinetics. The monolayer structures change as selenolate coverage increases, from being epitaxial and consistent with the initial thiolate structure to being characteristic of selenolate monolayer structures. At room temperature and at positive sample bias in scanning tunneling microscopy, the selenolate-gold attachment is labile, and molecules exchange positions with neighboring thiolates. The scanning tunneling microscope probe can be used to induce these place-exchange reactions

Directed Assembly of Functionalized Carborane Analogs

Controlling molecular building blocks and their placement at the nanoscale is an important issue for assembly from the bottom up. Manipulating single molecules on the surface using self-assembly can be used in creating novel molecular devices. Self-assembled monolayers (SAMs) form when molecules spontaneously assemble on a surface from either solution or vapor deposition. Cage molecules, specifically carboranethiols have many advantages such as rigid three-dimensional structures, high-stability to chemical and heat degradation, symmetry, rigidity, straightforward functionalization and controllable intermolecular interactions. Using the unique properties, we can fine tune SAMs and gain a fundamental chemical and physical understanding at the nanoscale. Assembling carboranethiols onto Au{111}, creates pristine monolayers with minimal defects and are made rigid through intermolecular interactions. Difunctionalized carboranes have gained interest due to the second thiol group. Assembling carboredithiol on Au{111} reveals a hexagonally close packed monolayer with two different intensity protrusions. We attribute these two protrusions as two distinct binding sites on the surface: with both thiols bound or one thiol bound and one unbound. Controlling the directionality of these binding sites is possible through protonation. Using strong acids and bases we can direct binding modalities in either direction. Functionalizing carboranethiols provides even greater tunability over the surface. P-carborane and its functionalized analog, p-mercaptobenzoic acid are analyzed using STM. These assemblies pack in a hexagonally close packed lattice which adsorb primarily as thiolates and thiols. Contact angle measurements confirm the hydrophilic character of p-mercaptobenzoic acid monolayers containing the carboxylic acid group. Mixed monolayers of p-carborane and p-mercaptobenzoic acid provide an excellent foundation for two and three dimensional structures. Using STM’s local barrier height (LBH) mode we can track dipoles on a surface. Assembling various carboranes with different dipoles allows us to visualize how these dipoles align and interact with neighboring molecules. Dipoles align based on intermolecular interactions with surrounding molecules and across different monolayer and surface defects, and locally align at low temperatures. Finally we look at place-exchange reactions involving alkanethiolates and alkaneselenoates through STM. Alkanthiolates are rapidly replaced by alkaneselenoates, as selenol coverage increases. The monolayer structure changes as selenoate coverage increases and with positive sample bias in STM, the selenolate-gold complex becomes labile and exchanges positions with neighboring thiolates

Exchange reactions between alkanethiolates and alkaneselenols on Au{111}.

When alkanethiolate self-assembled monolayers on Au{111} are exchanged with alkaneselenols from solution, replacement of thiolates by selenols is rapid and complete, and is well described by perimeter-dependent island growth kinetics. The monolayer structures change as selenolate coverage increases, from being epitaxial and consistent with the initial thiolate structure to being characteristic of selenolate monolayer structures. At room temperature and at positive sample bias in scanning tunneling microscopy, the selenolate-gold attachment is labile, and molecules exchange positions with neighboring thiolates. The scanning tunneling microscope probe can be used to induce these place-exchange reactions

Acid-Base Control of Valency within Carboranedithiol Self-Assembled Monolayers: Molecules Do the Can-Can.

We use simple acid-base chemistry to control the valency in self-assembled monolayers of two different carboranedithiol isomers on Au{111}. Monolayer formation proceeds via Au-S bonding, where manipulation of pH prior to or during deposition enables the assembly of dithiolate species, monothiol/monothiolate species, or combination. Scanning tunneling microscopy (STM) images identify two distinct binding modes in each unmodified monolayer, where simultaneous spectroscopic imaging confirms different dipole offsets for each binding mode. Density functional theory calculations and STM image simulations yield detailed understanding of molecular chemisorption modes and their relation with the STM images, including inverted contrast with respect to the geometric differences found for one isomer. Deposition conditions are modified with controlled equivalents of either acid or base, where the coordination of the molecules in the monolayers is controlled by protonating or deprotonating the second thiol/thiolate on each molecule. This control can be exercised during deposition to change the valency of the molecules in the monolayers, a process that we affectionately refer to as the "can-can." This control enables us to vary the density of molecule-substrate bonds by a factor of 2 without changing the molecular density of the monolayer

Exchange Reactions between Alkanethiolates and Alkaneselenols on Au{111}

When alkanethiolate self-assembled monolayers on Au{111} are exchanged with alkaneselenols from solution, replacement of thiolates by selenols is rapid and complete, and is well described by perimeter-dependent island growth kinetics. The monolayer structures change as selenolate coverage increases, from being epitaxial and consistent with the initial thiolate structure to being characteristic of selenolate monolayer structures. At room temperature and at positive sample bias in scanning tunneling microscopy, the selenolate–gold attachment is labile, and molecules exchange positions with neighboring thiolates. The scanning tunneling microscope probe can be used to induce these place-exchange reactions

Self-Assembled <i>p</i>‑Carborane Analogue of <i>p</i>‑Mercaptobenzoic Acid on Au{111}

The <i>p</i>-carborane cluster analogue of <i>p</i>-mercaptobenzoic acid, 1-HS-12-COOH-1,12-C₂B₁₀H₁₀, has been synthesized and characterized using nuclear magnetic resonance spectroscopy, single-crystal X-ray diffraction analysis, quantum-chemical calculations, and scanning tunneling microscopy. The single-crystal structure and selected packing aspects are discussed and presented in comparison with the two-dimensional periodic arrangements. Scanning tunneling micrographs, recorded under ambient conditions, are used to compare pure monolayers of 1-HS-1,12-C₂B₁₀H₁₁ to coadsorbed monolayers of both the parental precursor and carboxyl-functionalized <i>p</i>-carboranethiolate on Au{111}. Monolayers of both constituents are further characterized by X-ray photoelectron spectroscopy, which shows good agreement between the stoichiometry of each pure monolayer and the nominal stoichiometries of the respective molecules. Results indicate that most of the molecules of both derivatives adsorb as thiolates but that a small fraction of each adsorbs as thiols, without complete SH bond scission, and consequently are labile relative to desorption. Wetting-angle measurements confirm the hydrophilic character of monolayers containing the carboxylic acid constituents. Mixed self-assembled monolayers with functionalized constituents of high axial symmetry provide a convenient basis for grafting two- and three-dimensional structures

Defect-Tolerant Aligned Dipoles within Two-Dimensional Plastic Lattices

Carboranethiol molecules self-assemble into upright molecular monolayers on Au{111} with aligned dipoles in two dimensions. The positions and offsets of each molecule’s geometric apex and local dipole moment are measured and correlated with sub-Ångström precision. Juxtaposing simultaneously acquired images, we observe monodirectional offsets between the molecular apexes and dipole extrema. We determine dipole orientations using efficient new image analysis techniques and find aligned dipoles to be highly defect tolerant, crossing molecular domain boundaries and substrate step edges. The alignment observed, consistent with Monte Carlo simulations, forms through favorable intermolecular dipole–dipole interactions